Eventstream API light client events spec compatibility

[nflaig]

Describe the bug

Currently Lodestar does not return light client events as defined in spec examples beacon-APIs/#/Events/eventstream.

The spec example has an additional version field and wrapps the actual light client updates into data layer (similar to REST API)

            light_client_finality_update:
              description: The node's latest known `LightClientFinalityUpdate` has been updated
              value: |
                event: light_client_finality_update
                data: {"version":"phase0", "data": {"attested_header": {"beacon": { ... }}

This incompatibility causes issues as Lodestar and Nimbus beacon node return different data formats.

• https://lodestar-mainnet.chainsafe.io/eth/v1/events?topics=light_client_optimistic_update
• http://testing.mainnet.beacon-api.nimbus.team/eth/v1/events?topics=light_client_optimistic_update

This was brought up by @rdvorkin at the Light Client Summit at Devconnect in Building a trustless light-client assistant in your browser around 11:30

Other issues that @jeluard brought up about the light client event examples

• version is phase0 whereas light client was introduced in altair (fails our spec tests)
• sync_committee_bits too short

We should clarify if the current format as outlined in the spec should be the standard or we want to omit version and data layer. Other events do not use a data layer in the serialized json which makes the light client events a bit of an outlier.

Additional context

We currently just calculate the fork version based on slot in attested header

compute_fork_version(compute_epoch_at_slot(data.attested_header.beacon.slot))

lodestar/packages/api/src/beacon/routes/events.ts

Line 213 in c044d4c

getLightClientTypeFromHeader((data as unknown as allForks.LightClientOptimisticUpdate).attestedHeader)[

Related

Testing Lodestar events API against the spec examples causes issues as well and we have to ignore those topics during testing

• Support eventstream example (light_client_*_update) from recent beacon API test suite #6170

Beacon-API spec PR (there was no discussion around events format)

• Define endpoints for light client sync ethereum/beacon-APIs#247

[philknows]

We should aim to follow the spec here unless there's a reason for why we don't, so we can follow up with a potential spec change with some additional context.